Yoshihiro Ida

Image quality dependence on in-plane positions and directions for MDCT images

OBJECTIVE The present study was performed to examine the dependence of image quality on in-plane position and direction in computed tomography (CT) imaging using the modulation transfer function (MTF), noise power spectrum (NPS) and analysis of signal-to-noise ratio (SNR). For detailed analysis of SNR, the low-contrast detectability was compared using simulated small low-contrast objects. MATERIALS AND METHODS Three models of multidetector-row CT (MDCT) were employed. The measurement positions for MTF were set to the isocentre and several peripheral areas, and NPS and SNR were calculated for the isocentre and 128 mm off-centre. To evaluate directional dependence, the one-dimensional physical properties were measured separately in the radial and azimuthal directions. Seven radiological technologists also performed a perceptual detection study at the different in-plane positions using computer-simulated low-contrast images. RESULTS The results of MTF and SNR differed between the isocentre and the peripheral area. The MTF values also tended to decrease with distance from the isocentre, and the SNR values in the low frequency range for the peripheral area were superior to those for the isocentre. In the detection study, the low-contrast detectability in the peripheral area was 13-40% higher than the value in the isocentre. CONCLUSION The results of the present study indicated that clinical CT images have remarkable non-uniformity of image quality. Therefore, the detailed analysis performed in this study will provide useful information for the development of advanced image processing applications, such as computer-aided diagnosis (CAD) and de-noising of CT images.

Extraction and recognition of the thoracic organs based on 3D CT images and its application

A method for extracting and recognizing thoracic organ regions from three-dimensional (3D) CT chest images has been proposed in this paper. This method can be simply described as a pixel-labelling process, that is, each pixel in a chest CT image is attached with a predefined label that indicates a special organ or tissue of human body. The density distribution and strength of connectivity of different organ regions have been investigated and used for the recognition process. We developed a system to visualize the 3D CT images based on volume rendering technique and intergraded this method into it. We found that with the recognition results we can view the shape of a special organ without any overlap and can understand the relationship of human organs more clearly and easily.

New weighting factor of weighted CTDI equation for PMMA phantom diameter from 8 to 40 cm: A Monte Carlo study

Purpose: The weighted computed tomography dose index (CTDIw) uses measured CTDI values at the center and periphery of a cylindrical phantom. The CTDIw value is calculated using conventional, Bakalyars, and Chois weighting factors. However, these weighting factors were produced from only 16‐ and 32‐cm‐diameter cylindrical phantoms. This study aims to devise new weighting factors to provide more accurate average dose in the central cross‐sectional plane of cylindrical phantoms over a wide range of object diameters, by using Monte Carlo simulations. Methods: Simulations were performed by modeling a Toshiba Aquilion ONE CT scanner, in order to compute the cross‐sectional dose profiles of polymethyl methacrylate (PMMA) cylindrical phantoms of each diameter (8–40 cm at 4‐cm steps), for various tube voltages and longitudinal beam widths. Two phantom models were simulated, corresponding to the CTDI100 method and the method recommended by American Association of Physicists in Medicine (AAPM) task group 111. The dose‐computation PMMA cylinders of 1 mm diameter were located between the phantom surfaces and the centers at intervals of 1 mm, from which cross‐sectional dose profiles were calculated. By using linear least‐squares fits to the obtained cross‐sectional dose profiles data, we determined new weighting factors to estimate more accurate average doses in the PMMA cylindrical phantoms by using the CTDIw equation: CTDIw = Wcenter • CTDIcenter + Wperiphery • CTDIperiphery. In order to demonstrate the validity of the devised new weighting factors, the percentage difference between average dose and CTDIw value was evaluated for the weighting factors (conventional, Bakalyars, Chois, and devised new weighting factors) in each calculated cross‐sectional dose profile. Results: With the use of linear least‐squares techniques, new weighting factors (Wcenter = 3/8 and Wperiphery = 5/8 where Wcenter and Wperiphery are weighting factors for CTDIcenter and CTDIperiphery) were determined. The maximum percentage differences between average dose and CTDIw value were 16, −12, −8, and −6% for the conventional, Bakalyars, Chois, and devised new weighting factors, respectively. Conclusions: We devised new weighting factors (Wcenter = 3/8 and Wperiphery = 5/8) to provide more accurate average dose estimation in PMMA cylindrical phantoms over a wide range of diameter. The CTDIw equation with devised new weighting factors could estimate average dose in PMMA cylindrical phantoms with a maximum difference of −6%. The results of this study can estimate the average dose in PMMA cylindrical phantoms more accurately than the conventional weighting factors (Wcenter = 1/3 and Wperiphery = 2/3).

Pin-photodiode array for the measurement of fan-beam energy and air kerma distributions of X-ray CT scanners

PURPOSE Patient dose estimation in X-ray computed tomography (CT) is generally performed by Monte Carlo simulation of photon interactions within anthropomorphic or cylindrical phantoms. An accurate Monte Carlo simulation requires an understanding of the effects of the bow-tie filter equipped in a CT scanner, i.e. the change of X-ray energy and air kerma along the fan-beam arc of the CT scanner. To measure the effective energy and air kerma distributions, we devised a pin-photodiode array utilizing eight channels of X-ray sensors arranged at regular intervals along the fan-beam arc of the CT scanner. METHODS Each X-ray sensor consisted of two plate type of pin silicon photodiodes in tandem - front and rear photodiodes - and of a lead collimator, which only allowed X-rays to impinge vertically to the silicon surface of the photodiodes. The effective energy of the X-rays was calculated from the ratio of the output voltages of the photodiodes and the dose was calculated from the output voltage of the front photodiode using the energy and dose calibration curves respectively. RESULTS The pin-photodiode array allowed the calculation of X-ray effective energies and relative doses, at eight points simultaneously along the fan-beam arc of a CT scanner during a single rotation of the scanner. CONCLUSIONS The fan-beam energy and air kerma distributions of CT scanners can be effectively measured using this pin-photodiode array.

Analysis of in-plane signal-to-noise ratio in computed tomography

The purposes of this study are to analyze signal-to-noise ratio (SNR) changes for in-plane (axial plane) position and in-plane direction in X-ray computed tomography (CT) system and to verify those visual effects by using simulated small low-contrast disc objects. Three-models of multi detector-row CT were employed. Modulation transfer function (MTF) was obtained using a thin metal wire. Noise power spectrum (NPSs) was obtained using a cylindrical water phantom. The measurement positions were set to center and off-centered positions of 64mm, 128mm and 192mm. One-dimensional MTFs and NPSs for the x- and y-direction were calculated by means of a numerical slit scanning method. SNRs were then calculated from MTFs and NPSs. The simulated low-contrast disc objects with diameter of 2 to 10mm and contrast to background of 3.0%, 4.5% and 6.0% were superimposed on the water phantom images. Respective simulated objects in the images are then visually evaluated in degree of their recognition, and then the validity of the resultant SNRs are examined. Resultant in-plane SNRs differed between the center and peripheries and indicated a trend that the SNR values increase in accordance with distance from the center. The increasing degree differed between x- and y-direction, and also changed by the CT systems. These results suggested that the peripheries region has higher low-contrast detectability than the center. The properties derived in this study indicated that the depiction abilities at various in-plane positions are not uniform in clinical CT images, and detectability of the low contrast lesion may be influenced.

Precisely Targeted Tumor Biopsy and Marking under CT-Fluoroscopy

An innovative imaging procedure, computed tomographic (CT) fluoroscopy, was recently developed by Katada and Toshiba Medical Corporation in Japan [1,2]. This procedure permits sequential display of computed tomograms in real time. CT-fluoroscopy has been found to be extremely useful for real-time monitoring, which ensures the safety and accuracy of invasive techniques such as punctures in the neurosurgical field [3]. We recently, attempted targeted needle biopsy at three points, the subcortex near the lesion and the lateral side and center of the lesion, under CT-fluoroscopy. We then attempted to precisely place a marker at the inner border of the lesion near the internal capsule. A round mini-coil was used as the marker. Two weeks later we succeeded in extensively resecting the tumor lesion without producing motor weakness because the marker’s shadow on an ultrasonic image allowed navigation to the bottom of the lesion very precisely by showing where the eloquent area was located. A neuronavigation system using preoperative images was used simultaneously. There was a difference of 10 mm between the point indicated with the navigator and the actual position of the marker. This difference is considered to reflect brain shift caused by intraoperative leakage of cerebrospinal fluid. Thus, the computer-aided neuronavigation system appears to require a feedback technique for intraoperative brain shift that would enhance precision, reliability, and safety. In this respect, CT-fluoroscopy combined with intraoperative CT is considered to play a potentially very important role.